1. Field of the Invention
The present invention relates to methods of extruding a raw material to shape a honeycomb molded body and fire the honeycomb molded body to produce a honeycomb structural body.
2. Description of the Related Art
A honeycomb structural body has been widely known to support catalyst therein. The catalyst has a purification property. In general, such a honeycomb structural body has cell walls arranged in a lattice like shape and a plurality of cells surrounded by the cell walls. When the honeycomb structural body supports catalyst therein and is mounted on an exhaust gas pipe in an exhaust gas purification system of a motor vehicle, etc., the catalyst supported in the honeycomb structural body is activated at a predetermined high temperature by using exhaust gas emitted from an internal combustion engine passed through the exhaust gas pipe of the motor vehicle. The activated catalyst purifies the exhaust gas, and the purified exhaust gas is discharged to the outside of the exhaust gas purification system.
Recently, because the vehicle emissions control of reducing motor vehicle emissions, etc. is becoming stricter year by year in view of environmental protection, there is a strong demand to decrease carbon dioxide contained in exhaust gas emitted from an internal combustion engine and more improve fuel efficiency of motor vehicles. Thus, there is a strong demand for the honeycomb structural body to have an improved and excellent exhaust gas purification capability. In order to satisfy these recent demands, honeycomb structural bodies having various structures have been proposed. A conventional technique has been proposed with a honeycomb structural body having an improved structure in which an inside section and outside section have a different cell density. Another technique has disclosed another structure of a honeycomb structural body in which a boundary partition wall is formed between the inside section and the outside section in a cross section perpendicular to an axial direction of the honeycomb structural body. However, these techniques require a complicated design for producing a metal die for use in the production of such a complicated structure of the honeycomb structural body. Further, there is a possible drawback of breaking the metal die when the honeycomb structural body is produced, and generating defects of cells during the production of the produced honeycomb structural body. In fact, it is difficult to produce a honeycomb structural body having a complicated structure by using a metal die.
In order to solve such a conventional problem, a patent document, Japanese patent laid open publication No. H04-332604, discloses a method using a metal die having an improved structure in which an inside metal die having a small diameter is fitted to an outside metal die having a penetration hole.
However, in the structure of the metal die used in the patent document, a high local stress is generated the fitted section between the inside metal die and the outside metal die, and as a result, the metal die is broken by a high local stress when the metal die is used for producing a honeycomb structural body. In addition, it is generally difficult to correctly mate feeding holes formed in the inside metal die with feeding holes formed in the outside metal die, and to produce the metal die having such a complicated structure. That is, it is difficult to produce a metal die corresponding to a complicated and various types of cell structures of a honeycomb structural body.